“Optical communication ring network” here means not only a communication network configured as a whole as a ring but, in general, any section of a network comprising a plurality of nodes and branches and arranged, at least temporarily, in a ring configuration.
Specifically, the present invention tackles the problem of making a ring network with self-healing capabilities, i.e. capabilities of managing failures which may arise in the connections between the network nodes.
“Failure” here means any event which may affect the physical optical carriers (e.g. breakage or interruption of an optical fiber) and/or the devices of the network operating on the transmitted signals (e.g. receivers; demultiplexers, amplifiers, etc.), in such a way as to bring about a situation of degradation of the transmission which is deemed not tolerable; the term “failure” thus should in no way be interpreted as being limited only to events causing the complete interruption of the connection.
The state of the art includes many methods to optically protect a set of wavelength multiplexed optical channels carried by a two-fiber optical ring network.
European Patent Application No. 769859 in the name of the Applicant relates to a transparent optical self-healing-ring communication network in which at least two nodes are optically connected along a first and a second closed optical path having opposite transmission directions. Each node simultaneously feeds its transmitted signals to the first and the second closed optical path, so that the signals travel along complementary arcs of the ring. The receiving node selectively receives the signals from one of the two closed paths and, in case of failure on this closed path, switches reception on the other closed path. The Applicant observes that this Optical Channel Protection method, identifiable as a “Optical Channel 1+1 Dedicated Ring Protection” method, requires that, for each wavelength used on one closed path, the same wavelength cannot be used on the other closed path other than for protection. Thus, with N wavelengths, only N protected channels can be used in the network.
European Patent Application No. 97123013.1-2209 filed on Dec. 31, 1997 in the name of the Applicant relates to a method and an apparatus for transparent optical communication with two-fiber bidirectional ring with autoprotection and management of low priority traffic. The communication network comprises two optical fibers (an external one and an internal one) that interconnect the nodes. According to the proposed technique, if a first and a second node of the network are considered for mutual signal transmission, a high-priority traffic can be set up on a first bidirectional communication path defined between the two nodes on the external and internal fibers, using only an arc of the communication ring defined between the two nodes.
The arc complementary to that just described can be used as a second bidirectional communication path between the two nodes for low-priority traffic, using the same wavelength as the preceding channel. The protection mechanism consists of redirecting the high-priority traffic onto the second path in the case of a breakdown in, or degradation of, communication on the first path, thus losing the low-priority traffic on the second path. This protection method is identifiable as a “Optical Channel 1:1 Dedicated. Ring Protection” method with management of Low-Priority traffic. The Applicant observes that; although this method provides a double capacity with respect to the previous technique, with N wavelengths it is still not possible to protect more than N channels.
The article “A Transparent, All-Optical, Metropolitan Network Experiment in a Field Environment: The “PROMETEO” Self-Healing Ring”, F. Arecco et al., IEEE Journal of Lightwave Technology, Vol. 5, December 1997, describes a method to provide protection against failures in a metropolitan ring network. According to this method, a working fiber ring is used to carry all the optical channels between nodes, while a protection fiber ring is empty under normal operating conditions. In case of failure, a protection switching takes place at the nodes adjacent to the failure, which reconfigure in such a way to route all the optical channels (i.e. all the wavelengths of the multiplex section) through the protection fiber ring so as to bypass the failure. This protection method is known as “Optical Multiplex Section Protection” method. The Applicant observes that, once more, with N wavelengths it is possible to carry only N protected channels. Moreover, the Applicant further observes that, in this case, it is not possible to have different protection mechanisms (i.e. protection at different layers) for different channels, and that the optical path after a protection reconfiguration can be longer than the ring circumference.
U.S. Pat. No. 5,647,035 in the name of CSELT—Centro Studi e Laboratori Telecomunicazioni S.p.A.—provides a ring network communication structure on an optical carrier and a reconfigurable node for said structure. In said structure, a plurality of nodes are interconnected by means of connections comprising at least a first and a second optical carrier, such as an optical fiber. Transmission between two nodes occurs on the ring according to a WDM scheme, by utilizing a first wavelength for communication in one direction on the first carrier and a second wavelength for communication in the opposite direction on the second carrier. The second wavelength on the first carrier and the first wavelength on the second carrier are reserved for protection (protection channels) and are “shared” among all the nodes. Under regular operation conditions of the network, in each node the signals conveyed by the two fibers are detected, processed as required in units of a higher hierarchical level, converted again in optical signals and re-transmitted towards the following node. In the presence of a failure on one of the connections, the nodes adjacent to the failed connection reconfigure themselves to ensure the continuation of communication on the alternative path provided by the ring, by utilizing the first wavelength on the second carrier and the second wavelength on the first carrier. The exemplary embodiment described referring to just two wavelengths λ1,λ2, can be generalized to any number of wavelengths with a corresponding expansion of the described connection; switching matrices of the n×n type may be used.
The Applicant noticed that, in the ring network proposed in U.S. Pat. No. 5,647,035, if multi-wavelength signals have to be managed, since signal re-routing is localized at the nodes adjacent to the failure, the protection operations have to be performed on the entire set of wavelengths of the multiplex section (as described for SDH in ITU-T Recommendation G-803 and G-841) and the reconfigured nodes must re-route all the working channels previously sent on the damaged ring segment to their respective protection channels running onto the complementary ring arc. This technique is then identifiable as an “Optical Multiplex Section Shared Protection” technique. Thence, each node in the network must be equipped with the optical switching tools for the complete set of wavelengths in the ring, and a switch matrix is then needed with a complexity which increases considerably with increasing the number of channels (e.g. if each channel carries 2.5 Gb/s and the system is adapted to transmit 16 channels, each matrix must be able to switch 16×2.5 Gb/s). It can be demonstrated that, in this case, the required optical switching blocks to protect a number of links between N and MN/2 (where N is the number of wavelengths and M the number of nodes) is always MN.
Furthermore, the Applicant observes that, in the Optical Multiplex Section Shared Protection, since the failure control is performed at the multiplex section level, a failure on a single optical channel (e.g. when a node transmitter is damaged) can be either ignored or it can cause the reconfiguration of all the traffic thus causing a temporary failure on all the other working channels.
The Applicant further observes that, in case such a network is reconfigured as a consequence of a failure, the protection path of a generic optical channel may be longer than the maximum ring circumference. This can occur when a generic bidirectional link is set up between two non-adjacent nodes in the network, as illustrated in FIG. 1a (where nodes A and D are involved). In case of failure the switching action is performed by the two nodes adjacent to the failure (i.e. nodes B and C), as shown in FIG. 1b. Briefly, each channel of the bidirectional link travels along the working path from the source node to one of the reconfigured nodes where it is routed into the protection path. Then it travels in opposite direction along all the ring network to the other reconfigured node where it is routed again on the working path and, finally, it reaches the destination node. This alternative optical channel path may have a length exceeding the ring circumference, reaching values of several hundred kilometers and then introducing low levels of S/N ratio and high levels of attenuation. In case of a transoceanic application, this protection solution may lead, due to loopbacks, to restoration transmission paths that would cross the ocean three times.
To overcome this last problem, a different protection solution has been proposed, which is referred to as a “MS shared protection ring—transoceanic application” in the Annex A of the Telecommunication standard ITU-T Recommendation G.841. In brief, a failure is detected at the two nodes adjacent to the failure at the SDH multiplex section layer and, subsequently, the nodes terminating failed links are informed of the failure situation and re-route the corresponding links on the complementary ring arc path, as illustrated in FIG. 1c. In other words, in case of failure, all the transmission links affected by the failure are bridged at their source nodes onto the protection channels that travels away from the failure. When the affected links reach their final destination nodes, they are switched to their original drop point. Therefore, no loopbacks are established and there is no risk of having, in case of failure, restoration transmission paths crossing more times the ocean.
The Applicant observed that, in this case, as in the protection solution of U.S. Pat. No. 5,647,035, the failure control is over the entire optical multiplex section and, therefore, a failure on a single optical channel may be either ignored or it can cause the reconfiguration of all the traffic thus causing a temporary failure on all the other working channels.
The Applicant further observes that a protection at the SDH layer requires a relatively complex electronic layout and can operate only at a predetermined bit rate.